The invention relates generally to imaging systems and more specifically to a method and system for estimating and correcting time delays in an ultrasound imaging system.
Ultrasound systems comprise an array of transducer elements used for transmitting a set of waveforms into an imaging subject and for receiving a set of reflected ultrasound signals. Each waveform is emitted with a relative time delay chosen to focus the net transmitted waveform in a desired direction and depth and with a desired shape. Similarly each received signal is individually delayed to maximize the response of the system to reflected energy for a desired direction and depth and with a desired shape. The delayed receive signals are summed and processed to create and display an image of the imaging subject.
The transmit and receive time delays, known collectively as beamforming time delays, are typically calculated assuming that sound propagates through the body with a known, constant speed. When this assumption fails, the transmit and receive focusing is degraded and there will be a loss of image resolution and contrast.
One way to reduce this loss of image quality is to adjust the beamforming time delays based on measurements of the relative time delays of the receive signals. It is convenient to measure these relative time delays after the receive beamforming delays have been applied to them. If the assumption of a known, fixed sound speed is correct, the delayed receive signals will be well-aligned in time, i.e., the arrival time errors will be small. If the assumption is not correct, the delayed receive signals will not be well-aligned in time; the arrival time errors will be large. By adjusting the beamforming delays for the arrival time errors, the focusing will be improved and image resolution and contrast will increase.
In medical ultrasound imaging, the estimation of arrival time errors must be fast, accurate and robust. It is also very desirable that the extra cost required to implement the estimation hardware be minimized.
A fast estimation is desired because the beamforming corrections need to be updated quickly since the required corrections will vary as the transducer moves, either as the operator moves the probe over the patient as part of the normal scanning procedure, or due to slight movement of the operator's hand or because of patient motion or breathing.
An accurate estimation is desired to improve image resolution and contrast and to avoid undesirable degradation of the image due to the adjustment of beamforming time delays by incorrect time delay estimates. Beamforming time delay errors typically introduce artifacts into the image which may lead to incorrect diagnosis or a longer examination time. The rate of artifact production must be sufficiently low for the majority of operators to routinely use the time delay correction feature and thereby gain the benefit of improved image resolution and contrast.
The Fourier spectrum of a real signal with a bandwidth which is not too large (as is typical for ultrasound signals) consists of two relatively isolated regions of non-negligible amplitude, known more commonly as bands. One of these bands is centered around a positive frequency known as the “carrier” frequency, and the other band is centered around a negative frequency which is the opposite of the carrier frequency. There are many methods of producing the baseband signal corresponding to a real signal but the desired net effect is to suppress the negative frequency band and to shift the positive frequency band in frequency such that it is approximately centered at zero frequency. Note that the baseband signal is complex.
A related signal to the baseband signal is the analytic signal. Mathematically, the analytic signal is derived from a real signal by removing its negative frequency components. In practical systems, the negative frequency components are suppressed, but not totally eliminated, by filtering. The analytic signal differs from the baseband signal in that the positive frequency spectral band is not shifted down in frequency such that it becomes centered at zero frequency.
One method to estimate time delays between two real signals requires converting both signals to their complex baseband form. The complex conjugate of one baseband signal is multiplied sample by sample with the other baseband signal and then summed. The phase of the resulting complex number is proportional to the time delay error between the two signals. One problem with the above method is the requirement for converting both real signals to complex form. Converting signals to their baseband form requires large and expensive filters. Since this method requires converting each receive signal to its baseband form, it is undesirably costly. It is helpful to consider the above method as follows. Let SB0(t) and SB1(t) be two baseband signals, each a function of time t. For simplicity, t is considered to be a continuous variable. In practice, the signals are sampled over a set of evenly spaced time intervals, t[i]=iΔt, where Δt is the sampling time interval.
The method described above constructs a complex correlation sum ‘C’ by integrating the product of one baseband signal with the complex conjugate of the other baseband signal as shown in the equation below.C=∫−∞+∞dtS*B0(t)SB1(t)  Equation (1)As is well-known, however, the integral over time (as in equation 1) can also be expressed as the integral over frequency of the spectra of the two signals:C=∫−∞+∞dtS*B0(t)SB1(t)=∫−∞+∞dfA*B0(f)AB1(f)  Equation (2)In Eq. (2), AB1(f) is the Fourier transform of the baseband signal SB1(t), and A*B0(f) is the complex conjugate of the Fourier transform of the baseband signal SB0(t).
The above described method of estimating time delay is usually accurate when the signals which are compared are produced by relatively uniform random scatterers. An example of this in the human body is a region of the liver without bright arterial walls and without large, nearly anechoic, blood vessels. In practice, such a region of uniform scatterers is not always available. As a result, the time-delay estimates can be corrupted, especially when there are strongly reflecting scatterers which are not aligned with the desired scan direction. A strongly reflecting, off-axis scatterer produces a signal at the transducer with an arrival time error which varies approximately linearly across the array. If such a signal were used to estimate the time delay error, then correcting the observed arrival time errors would erroneously steer the beamformer toward the scatterer.
Therefore there is a need for a method and system in ultrasound systems to accurately estimate time delays while minimizing the cost and size of the system.